1. Field of the Invention
The present invention relates to an AM radio receiver provided with functions for detecting and eliminating noise superimposed on a received radio station signal.
2. Description of the Related Art
Noises such as pulse noise in an AM radio receiver are caused mainly by ambient electrical and mechanical disturbances entering the radio station signal from the antenna, and in an AM radio receiver that employs an envelope detection, any superimposed disturbance pulses are detected as audio, causing an annoying sound quality.
In automobile AM radios in particular, ignition noise generated from the engine's spark plugs and other disturbances generated by the mechanical operation of other electrical systems are superimposed from the antenna, remarkably affecting the audio.
An example of an AM radio receiver that has functions for detecting and eliminating ambient noise is described in U.S. Pat. No. 4,856,084. Such a conventional AM radio receiver will be described hereinafter with reference to FIG. 9.
This AM radio receiver is composed of an antenna 1, a RF amplifier 2, a first mixer 3, a local oscillator 4, a first bandpass filter 5, a second mixer 6, a crystal oscillator 7, a second bandpass filter 8, an intermediate-frequency amplifier 9, an AM detector 10, an AM detector output terminal 11 and a noise detector 20. The operation of this AM radio receiver is as follows.
In FIG. 9, a radio station signal received by antenna 1 is amplified by the RF amplifier 2, and mixed in the first mixer 3 with a local oscillation signal from the local oscillator 4 so as to be converted to a first intermediate-frequency signal. The first intermediate-frequency signal is band-limited by the first bandpass filter 5, and mixed in the second mixer 6 with a crystal oscillation signal from the crystal oscillator 7 so as to be converted to a second intermediate-frequency signal. The second intermediate-frequency signal is band-limited by the second bandpass filter 8, amplified by the intermediate-frequency amplifier 9, AM-demodulated in the AM detector 10 and output from the output terminal 11.
On the other hand, the noise detector 20 detects noise superimposed at the antenna 1 from the output signal of the RF amplifier 2, and upon detection of noise, generates a blanking pulse for a certain period of time. During the period in which this blanking pulse is generated, the second mixer 6 and AM detector 10 are halted. During the period in which the second mixer 6 and AM detector 10 are halted, no noise is present in the radio station signal, so that the annoying sound is eliminated and the sound quality is improved.
However, with the conventional AM radio receiver constituted as shown in FIG. 9, since a single noise detector 20 halts both the second mixer 6 and the AM detector 10 for a certain period of time, a redundant operation is caused in such a manner that the AM detector 10 is halted even though the noise is eliminated by the second mixer 6. As a result, there is a problem that demodulation cannot be done with good sound quality as described below.
FIG. 10 illustrates the relationships among the radio station signal (a), the second mixer output signal (b), the second bandpass filter output signal (c), and the AM-demodulated signal (d), when both the second mixer 6 and the AM detector 10 are halted by a single noise detector 20 for a certain period of time. In the figure, the horizontal axis represents time and the vertical axis represents the potential. Noise components entering from the antenna are superimposed on the radio station signal (a).
During the noise elimination period in which noise is eliminated by the blanking pulse from the noise detector 20, no signal is output from the second mixer output signal (b). However, a waveform change occurs in the second bandpass filter output (c) due to the wave filter effect of the second bandpass filter 8, so that even if demodulation is performed by the AM detector 10 in this state, the AM-demodulated signal (d) has its waveform improved as indicated by the solid line Wa, and the sound quality deterioration is also improved.
However, when the AM detector 10 is also halted at the same time, a period of no signal output occurs in the AM-demodulated signal (d), and so the waveform is distorted as indicated by the dashed line Wb, so that the sound quality deteriorates.